Method for drying a product using a regenerative adsorbent

ABSTRACT

The invention relates to a method and an apparatus for drying a product using a regenerative adsorbent, more in particular to such a method which can be carried out in an energy-saving manner. According to the invention, the product is dried by bringing it into contact with an adsorbent, water being taken up form the product by the adsorbent. Subsequently, the adsorbent is regenerated with superheated steam, steam being obtained, which steam comprises at least a part of the mentioned water.

The invention relates to a method for drying a product using aregenerative adsorbent, more in particular to such a method which can becarried out in an energy-saving manner. The invention also relates to anapparatus suitable for carrying out such a method.

A method for drying water-containing products using an adsorbent is, forinstance, described in WO-A-00/02822. According to this known method, anadsorbent in the form of zeolite A is brought into contact with thewatery substance to be dried, which may or may not first be dewateredand/or preheated. In this method, the zeolite adsorbs water from thesubstance, while adsorption heat is produced, so that more waterevaporates. Subsequently, the thus obtained dried substance is separatedfrom the zeolite, and the zeolite is regenerated by heating in an oven.

A drawback of this known method is that in the regeneration step asubstantial amount of heat must be supplied to regenerate the zeolite.

The present invention contemplates providing a method for drying aproduct which can be carried out in an energy-saving manner.

It has been found that if the regeneration step is carried out usingsuperheated steam, a number of advantages is obtained. Therefore, thepresent invention relates to a method for drying a product, comprisingthe steps of:

-   -   bringing the wet product into contact with an adsorbent, water        being taken up from the product by the adsorbent;    -   regenerating the mentioned adsorbent with superheated steam,        steam being obtained, which steam comprises at least a part of        the mentioned water.

The wet product may be either a final product to be dried, such asfoods, for instance starch or carrots, or chemical products, forinstance pigments or cellulose derivatives and sludges, which are mixedwith the adsorbent (direct drying); or a wet gas stream originating froma previous drying step (indirect drying).

With the method of the invention, inter alia a product stream of steamis obtained, which stream comprises the water removed from the driedproduct by means of the adsorbent in the form of steam. An importantadvantage is that this steam stream can be employed for the purposes loconventional in the (agro-)process industry, for instance for heating.If, for instance for the regeneration step, the starting steam stream isa steam stream of 5 bara/160° C. (slightly superheated), this stream mayfirst be superheated to the desired temperature, for instance 300° C.Subsequently, with this superheated steam, the regeneration step iscarried out, while the temperature of the steam falls to, for instance,160° C. Because the water is released from the adsorbent in the form ofsteam, the amount of steam obtained with this released part increases.In this manner, therefore, a steam stream of 150° C./5 bara is obtained,which is, for instance, 105 wt. % of the original ingoing steam stream.This steam has sufficient energy content to be employed elsewhere forprocess-technological purposes.

With special advantage, the invention can be used for drying a gasstream originating from a drying step, preferably from a low temperaturedrying step. This embodiment is particularly suitable iftemperature-sensitive products, such as specific foods, must be dried.According to this embodiment, the food or other product to be dried isbrought into contact with a suitable gas, such as nitrogen, or if theproduct allows it, air, at a suitable temperature. The temperature mustbe selected such that, on the one hand, effective drying is obtainedand, on the other hand, the food or other product to be dried does notundergo appreciable thermic degradation. This product temperature istypically 30-90° C., for instance 60° C., for drying starch. Of course,this temperature differs per case. Thus, the method according to theinvention is also very suitable for freeze drying products, the producttemperature typically ranging between −40° C. and −10° C.

The low temperature drying step can be carried out in known per seapparatuses suitable to that end, for instance in a column. According tothe invention, the wet gas is then brought into contact with theadsorbent, water being taken up from the wet gas by the adsorbent and adry gas stream being obtained again, which dry gas stream can be appliedagain in the low temperature drying step. The water-loaded adsorbent isregenerated again with superheated steam, in the above-described manner.

An advantage of this embodiment, in which a wet product is first driedwith a gas stream, after which this gas stream is dried with aregenerative adsorbent, compared to bringing the food or other productto be dried directly into contact with the adsorbent, is that theadsorbent need not be separated from the food (or other product).

It is particularly advantageous if the embodiment is applied when freezedrying products.

It is particularly advantageous if in this embodiment the step ofbringing the wet gas stream into contact with the adsorbent is carriedout under such conditions that in this step a dry gas is obtained with atemperature suitable to be directly applied in the low temperaturedrying step, that is to say without first needing to cool or heat thisgas stream. To obtain a dry gas of a desired temperature, processfactors, such as temperature of the adsorbent, ratio of adsorbent to wetgas and contact time, can be varied. It is noted that through thereleased adsorption heat the gas heats up.

Also, it is possible to apply a combination of direct and indirectdrying, as indicated in FIG. 1. In this figure, the product to be dried(1), for instance a slurry of lactose, is introduced into a spray tower(2) via a sprayer. The drying medium (for instance nitrogen) circulatescountercurrently with the sprayed slurry, so that a first drying step iseffected. The wet drying medium is then dried via an adsorbent (4), asindicated. The thus obtained wet adsorbent is dried by steam, asdescribed above, steam being obtained which comprises the water.

The product (6) at the bottom of the column may, if desired, besubjected to a second drying step according to the invention, in whichit is, for instance, mixed with another adsorbent (7), as indicated inFIG. 1. The dried product is then separated from the adsorbent (7), forinstance by means of sieving or other separation techniques for solidsubstances. In this step, a wet stream of adsorbent is obtained, which,as described above, is dried by steam. Also, a dry product stream (9) isobtained, which product has therefore undergone two drying stepsaccording to the invention.

As adsorbent, the conventional adsorbents may be applied. Examplesthereof are silica gel, activated alumina, activated carbon, carbonmolecular sieves and zeolites. Preferably applied is an adsorbent whichcomprises a zeolite. Surprisingly, it has been found that in contrast towhat is generally assumed in the literature by Ruthven, D. M., 1984,Principles of Adsorption and Adsorption Processes, Chapter 1.5, JohnWiley & Sons, New York, no degeneration of the zeolite structure occursif this is brought into contact with superheated steam of temperaturessuitable according to the invention. At very high temperatures,degeneration of the zeolite can occur. For this reason, the temperatureof the superheated steam preferably ranges between 100 and 450° C., withmore preference between 150 and 400° C. With special preference, theadsorbent applied is one or more zeolites of the 3A, 4A and/or 5A type.

Because according to the invention the removed water is obtained in theform of HP/HT steam, a substantial saving in the energy cost can beobtained, which can rise to as much as 70%.

As stated, the obtained HP/HT steam stream can be employed for differentapplications, such as heating purposes. Also, it is possible to return apart of the obtained steam stream, after heating, again to the mentionedregeneration step for regenerating the adsorbent.

An apparatus for carrying out a method according to the inventioncomprises a space for bringing a product into contact with an adsorbent;and a space for regenerating the mentioned adsorbent with superheatedsteam, steam being obtained, which steam comprises at least a part ofthe mentioned water.

The invention can be applied for drying different products. It can bevery suitably applied for drying foods or starting materials in the foodindustry, such as, for instance, starch, starch derivatives, proteinsand fibers originating from starch-containing crops.

The invention can also be used for freeze drying products, in particularfoods. With reference to FIG. 2, the freeze drying of a product such as,for instance, carrots, can be carried out as follows.

1. Steam Drying

During the first phase, both the adsorbent (for instance zeolite) andthe carrots are dried. This can be done in series or parallel. Thecarrots are dried so far that the texture of the product is retained. Toretain the quality of the carrots, it is important that no air ispresent in the system at the end of this phase. The pressure duringsteam drying is atmospheric or less.

2. Vacuumizing

By condensing the steam in the carrot vessel, a reduced pressure will beformed. As a result, water will evaporate from the product, so that theproduct dries further and cools to about 20° C. By cooling the zeolitevessel, a reduced pressure will be formed here as well.

3. Freeze Drying

By mixing the carrots with the zeolite, a deeper vacuum will be formed,so that the water in the product will freeze. The frozen product willstill be dried, the sublimation heat is supplied through the adsorptionheat of the zeolite. The zeolite thereby remains at about 20° C., sothat the high adsorption capacity is retained. If required, externalcooling or heating may be effected to maintain the temperature. Theamount of needed zeolite can be calculated on the basis of the amount ofmoisture to be removed from the product.

1. A method for drying a product, comprising the steps of: bringing saidproduct into contact with an adsorbent, water being taken up from theproduct by the adsorbent; regenerating said adsorbent with superheatedsteam, thus providing a second steam, which second steam comprises atleast a part of said water.
 2. A method according to claim 1, whereinsaid product is a gas stream originating from a low temperature dryingstep.
 3. A method according to claim 1, wherein said adsorbent comprisesa zeolite.
 4. A method according to claim 3, wherein the zeolite is a3A, 4A and/or 5A zeolite.
 5. A method according to claim 1, wherein saidsecond steam has a temperature of at least 120° C.
 6. A method accordingto claim 1, wherein said second steam has a pressure of at least 3 bara.7. A method according to claim 1, wherein said product is freeze dried.8. An apparatus for carrying out a method according to claim 1,comprising a space for bringing a product into contact with anadsorbent; and a space for regenerating said adsorbent with superheatedsteam, thus providing a second steam, which second steam comprises atleast a part of said water.